US 6487252 B1 Abstract An orthogonal frequency division multiplexed wideband communication system provides improved time and frequency synchronization by inserting an unevenly spaced pilot sequence within the constellation data. A receive correlates the received data using the unevenly spaced pilot sequence. The pilot sequence is generated with a maximum length pseudo random noise code and inserted into frequency bins having prime numbers.
Claims(19) 1. A wireless communication system comprising:
a transmitter and a receiver coupled by an communication link, the transmitter comprising:
a modulator element for converting digital data to symbols, said symbols being assigned to frequency bins of a set of frequency bins;
a pilot tone generator for generating a sequence of pilot tones, each pilot tone being associated with frequency bins not assigned to symbols from said modulator element, the frequency bins assigned to said pilot tones being unevenly spaced therebetween;
a polyphase filter for performing an inverse fast Fourier transform (FFT) on the symbols assigned to said set frequency bins and providing a plurality of output samples;
a transmitting portion for transmitting the plurality of output samples over the communication link;
the receiver comprises a correlator for correlating the plurality of output samples received over the communication link with a reference for use in retrieving said digital data, said reference comprising said sequence of pilot tones associated with the unassigned frequency bins; and
the pilot tone generator includes a pilot tone location assignment table identifying said frequency bins assigned to each pilot tone.
2. A system as claimed in
3. A system as claimed in
4. A system as claimed in
5. A system as claimed in
6. A system as claimed in
means for adding a stepped frequency offset said reference producing a frequency offset reference;
means for correlating the plurality of output samples received over the communication link with each stepped frequency offset reference for use in retrieving said digital data.
7. A system as claimed in
a first FFT element for performing a course Fourier transform on the plurality of output samples received over the communication link, the first FFT element providing a time delay;
a delay element for delaying the plurality of output samples received over the communication link by said time delay;
a second FFT element for performing a fine Fourier Transform on the delayed plurality of output samples; and means for correlating the delayed plurality of output samples received over the communication link with said reference to retrieve said digital data.
8. A system as claimed in
9. A system as claimed in
a FFT element for performing a Fourier transform on correlated data provided by said correlator and providing channelized data;
a demodulator for operating on the channelized data, the channelized data substantially corresponding with said parallel digital data converted by said modulator.
10. A system as claimed in
11. A system as claimed in
a serial to parallel converter for converting sequential digital data from sequential to parallel digital data, and providing said parallel digital data to said modulator element;
a parallel to serial converter for converting the plurality of output samples provided by said polyphase filter to serial digital data, said transmitting portion transmitting said serial digital data; and
a multi-path formatter for adding a cyclic extension to said serial digital data prior to transmission by said transmitting portion.
12. A system as claimed in
the transmitter further comprises a differential encoder for differentially encoding the plurality of output samples provided by the polyphase filter, and
the receiver further comprises a differential decoder for differentially decoding the serial digital data received over the communication link and providing differentially decoded data, said differentially decoded data being corrected for frequency offset associated with said serial digital data received over the communication link.
13. A method of communicating comprising the steps of:
converting digital data to symbols, said symbols being assigned to frequency bins of a set of frequency bins;
generating a sequence of pilot tones, each pilot tone being associated with frequency bins not assigned to symbols from said modulator element, the frequency bins assigned to said pilot tones being unevenly spaced therebetween;
performing an inverse fast Fourier transfer (FFT) on the symbols assigned to said set frequency bins and providing a plurality of output samples; and
transmitting the plurality of output samples over a communication link;
correlating the plurality of output samples received over the communication link with a reference for use in retrieving said digital data, said reference comprising said sequence of pilot tones associated with unassigned frequency bins; and
the generating step includes the step of reading a pilot tone location assignment table identifying said frequency bins assigned to each pilot tone.
14. A method as claimed in
15. A method as claimed in
16. A method as claimed in
17. A method as claimed in
18. A method as claimed in
adding a stepped frequency offset said reference producing a frequency offset reference;
correlating the plurality of output samples received over the communication link with each stepped frequency offset reference for use in retrieving said digital data.
19. A method as claimed in
performing a course Fourier transform on the plurality of output samples received over the communication link to provide a time delay;
delaying the plurality of output samples received over the communication link by said time delay;
performing a fine Fourier Transform on the delayed plurality of output samples; and
correlating the delayed plurality of output samples received over the communication link with said reference to retrieve said digital data.
Description This invention relates in general to the field of communication systems, in particular to orthogonal frequency division multiplexed (OFDM) communication systems and more particularly to an improved synchronization of OFDM communication systems. Transmission of wideband digital data has become necessary due to evolving standards for high definition television (HDTV), digital video broadcasting, and wideband data networks. Orthogonal Frequency Division Multiplexing (OFDM) is a way to enhance the performance of wideband wireless communication links degraded by co-channel interference, impulsive noise, and frequency-selective fading. In the past, implementation complexity slowed the development of OFDM for useful commercial and handheld applications. With recent advances in semiconductor processing technology and digital signal processing, OFDM is now practical for system solutions including wireless LANs, audio and television broadcast radio links, and land mobile services. With the recent emergence of wideband cellular, and global satellite networks, a system and method are needed to provide synchronization of wideband wireless transmitted digital data which can be multiplexed onto a multicarrier waveform in a spectrally efficient manner. What is also needed is a system and method for digital synchronization that provides improved accurate timing and frequency tracking estimates for severely degraded and noisy channel environments. What is also needed is a method and system for synchronization correcting large timing misalignment of received multi-carrier waveforms, and provides acquisition of frequency offsets on the order of the RF signal bandwidth. The invention is pointed out with particularity in the appended claims. However, a more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the figures, wherein like reference numbers refer to similar items throughout the figures, and: FIG. 1 is a simplified functional block diagram of a wideband data transmitter in accordance with a preferred embodiment of the present invention; FIG. 2 is a simplified pilot sequence and assignment procedure in accordance with a preferred embodiment of the present invention; FIG. 3 is a simplified functional block diagram of a wideband data receiver in accordance with a preferred embodiment of the present invention; FIG. 4 is a simplified functional block diagram of a correlator in accordance with a preferred embodiment of the present invention; FIG. 5 is a simplified illustration showing pilot sequence insertion within constellation data in accordance with a preferred embodiment of the present invention; FIG. 6 is a simplified synchronizer system functional block diagram in accordance with the embodiments of the present invention; and FIG. 7 illustrates an example of window lengths used for prediction of current and future offsets in accordance with a preferred embodiment of the present invention. The exemplification set out herein illustrates a preferred embodiment of the invention in one form thereof, and such exemplification is not intended to be construed as limiting in any manner. The present invention provides, among other things, an accurate, efficient, and robust (e.g., operates in high receiver noise and fading) communication system and method of synchronization suitable for wireless reception of wideband and narrowband digital data. It may be implemented along with multiple popular modem waveforms, such as MPSK, MQAM, MRSK, and even MPPM, and is especially useful for video and wideband wireless networking applications. Processing complexity is minimized, for example, by deriving timing and frequency error estimates from similiar processes, making it suitable to handheld, portable devices, such as radios. The estimation accuracy of the present invention is suitable for very low signal-to-noise ratios (SNR's). In the preferred embodiments, converging of the process occurs within one baud interval, which is suitable for systems requiring rapid time and frequency acquisition and symbol timing and oscillator frequency alignment at system startup. The method of the present invention may be implemented, for example, with fully digital receivers or with receivers using analog down-conversion circuitry. The low-complexity, high-accuracy, rapid, and robust synchronization method of the present invention helps provide a quality of service suitable for wideband transmission of diverse digital information for the various applications. FIG. 1 is a simplified functional block diagram of a wideband data transmitter in accordance with a preferred embodiment of the present invention. Wideband transmitter Polyphase filter Parallel to serial converter converts each block, vector, or group of the digital samples provided by polyphase filter Transmitter elements FIG. 2 is a simplified pilot sequence and assignment procedure in accordance with a preferred embodiment of the present invention. Task In task In task In one preferred embodiment of the present invention, the number of pilot tones is determined dynamically in response to channel characteristics such that the number of pilot tones is increased, for example, when the signal to noise ratio decreases or the bit error rate increases. On the other hand, the number of pilot tones is reduced dynamically with improved signal to noise ratio or reduced bit error rates. This is discussed in more detail below. Polyphase filter FIG. 3 is a simplified functional block diagram of a wideband data receiver in accordance with a preferred embodiment of the present invention. Wideband receiver Correlator In one preferred embodiment of the present invention, a frequency offset inducer FIG. 4 is a simplified functional block diagram of a correlator in accordance with a preferred embodiment of the present invention. Correlator Time delay element FIG. 5 is a simplified illustration showing pilot sequence insertion within constellation data in accordance with a preferred embodiment of the present invention. FIG. 5 illustrates constellation data In accordance with a preferred embodiment, one function of the system of FIG. 1 is to convert digital data from a digital data source (or multiple sources), such as a speech, image, or video coder, and appropriately modulate and multiplex the symbols from the modulator so that the information may be transmitted over a RF channel. FIG. 1 illustrates one embodiment of a transmitter that includes OFDM waveform generator implemented using a polyphase filter structure. The modulator Pilot symbols, for example, aid in the synchronization processes for symbol timing (acquisition), clock recovery offset, frequency tracking, and frequency acquisition. A pilot sequence is inserted into the multiplexor along with the modulator data output (i.e. constellation data) (See FIG. For example, if the correlation function of a real (or complex) sequence P( then and where P A typical OFDM block size is N=1024, and a typical total pilot overhead would be approximately 4.1%. Generally, the synchronization process for any system, independent of the multiplexing method, uses a frequency synchronization process and a time synchronization process. At system power up, nothing may be known about timing or frequency error. The synchronization method of the present invention provides convergence in the presence of initial timing- and frequency-errors. Assuming frame timing is an independent process, although frame timing is easily accommodated, an OFDM time synchronizer aligns the polyphase filter window with the samples of the received signal. The primary role of a frequency synchronizer is to estimate frequency error in the received signal. This estimate is used, for example, to adjust the receiver local oscillator. This is important for OFDM systems because frequency error degrades the orthogonal properties between sub-channels (sub-carriers) causing inter-carrier interference. Frequency synchronization is performed in two steps: 1) frequency acquisition, and 2) frequency tracking. Frequency acquisition is typically more complex than frequency tracking, due to the capability to estimate large frequency error. FIG. 6 is a simplified synchronizer system functional block diagram in accordance with the embodiments of the present invention. In reference to FIG. 6, down-sampling and clock recovery are performed functionally in element The time and frequency estimation processes of the present invention, for example, take advantage of a-priori knowledge of the synchronization (pilot) sequence. The repetitive properties of the transmitted symbols, for example the correlative properties between the polyphase filter output symbol and cyclic extension may also be used. Y(n) is the received multi-carrier symbol, and the synchronization processes depend primarily on the computation of the following discrete correlation function in time: where the bar over p(n) indicates periodicity, denoted as {overscore (p)}(n), or N is the number of sub-carriers in each transmitted symbol, N where Y(k)=F[y(n)] and {overscore (P)}(k)is the pilot sequence with the bar indicating P(k) with periodicity, and “F” is the Fourier transform operator. These relationships are utilized for each of the synchronization processes described below. The discrete lag of the maximum magnitude of r(m) is used to estimate the symbol alignment error. In other embodiments, the maximum of the real part of r(m) is used to estimate the symbol alignment error. The discrete Fourier transform (DFT) of the correlation function is a product of two DFTs, and when eq. 4 represents the correlation, then
The bar represents the periodic extension. Using eq. 7 the symbol acquisition estimation process is equivalent to
where mag(·) represents the magnitude, and F As the samples of y(n) are received, any carrier frequency error f φ( where T
where θ represents an all zeros vector at each index n within the function φ(f The phase of TA (eq. 8) equals the averaged phase shift between the received samples of y(n) and the pilot sequence. Eq. 10 can be rewritten as Eq. 11 represents the sum of all the phase errors for n=1, 2, . . . N−1. Dividing this result by (N−1) provides the average phase shift. Using the sum of powers of the first n integers, eq. 11 can be greatly simplified. We can equate the simplified result to ∠TA which gives where f Eq. 12 is then able to provide a tracking range of approximately f For frequency acquisition, the discrete lag of the maximum magnitude of S(
With a maximum FA capture range on the order of N sub-carrier spacings, the course acquisition estimate, f
where FA represents the course offset as an integer multiple of the sub-carrier spacing. f For time tracking, as the samples of Y(k) are received, where Y(k)=F[y(n)], any fractional timing error t
where T
where Λ represents an all zeros vector at each index k within the function ψ(t The phase of FA (eq. 8) equals the averaged phase shift between the received samples of Y(k) and the pilot sequence. Eq. 10 can be rewritten as Eq. 18 represents the sum of all the phase errors for k=1, 2, . . . N−1. Dividing this result by (N−1) provides the average phase shift. The sum of powers of the first k integers, in eq. 18 is preferably used. The simplified result for ∠TA gives where t Eq. 20 is able to provide a tracking range of approximately T Because of the structure of the design of this synchronization method, there are many adaptive properties that require some clarification. First, based on the pilot insertion structure shown in FIG. 1, during extremely low received signal levels, additional performance can easily be accommodated by a modest increase in the pilot overhead. This can be a purely variable parameter by providing channel quality estimates to the transmitter as estimated by the receiver. One embodiment provides a dynamically changing overhead. This provides at least two advantages, it maximizes the data throughput for good channels and optimizes the needed amount of reference information for received signals under low channel SNRs. FIG. 7 illustrates an example of window lengths used for prediction of current and future offsets in accordance with a preferred embodiment of the present invention. FIG. 7 shows an example of y(n) with multiple measurement interval lengths is described in accordance with a preferred embodiment of the present invention. Although this has been described for a time-domain signal, the multi-resolution embodiment applies to the frequency domain estimation processes as described above. One purpose of the multi-resolution approach is to provide both shorter- and longer-term averages of the timing and frequency offset errors. The appropriate resolution is chosen given the drift characteristics of the system for both timing and frequency. This multi-resolution embodiment then lends itself to provide intra-signal correlation, that is the estimator takes advantage of correlation in the oscillator and timing clock drift properties within a signal and between received signals. An estimator operating on the received signal is utilized to confirm inter-signal drift correlation on the shorter signals. In a similar way an estimator operating on a longer received signal is utilized to confirm any inter-signal drift correlation on the signals represented in the received time domain signal. FIG. 7 An overlapping windowed structure is preferably used to ensure that the long-term estimator updates the estimate for each multi-carrier signal y One embodiment of the present invention includes an estimator predictor. One primary purpose of the predictor is to minimize the impact of wideband fading and severe signal loss due to propagation impairments. In the event of very low received SNR, the correlation process may be insufficient to provide an accurate estimate for the currently received signal. The predictor can, therefore, provide an estimate of the current (and future) timing and frequency offset, independent of the estimation of the correlation processes from the currently received signal. The current estimate can be viewed as a weighted average of past measured offset values, while future estimates can be predicted based on a weighted average of current and past values. In general, a polynomial predictor is preferred for drift characteristics which exhibit nonlinear tendencies with both positive and negative slopes. For this condition a curve fit is performed such that an error measure is minimized. For methods which utilize least squares curve fitting, a set of data points (t where: W P=the number of data points (t M=the order of the polynomial fit. C EN Note that a similar relation can be developed for time tracking (TT). The weight values (We) are used to weight the squared error for each data sample. The W The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and therefore such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Accordingly, the invention is intended to embrace all such alternatives, modifications, equivalents and variations as fall within the spirit and broad scope of the appended claims. Patent Citations
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